1. Field of the Invention
The present invention relates to a cleaning apparatus for cleaning workpieces, such as semiconductor wafers or glass substrate for LCDs.
2. Description of the Related Art
Generally, a cleaning apparatus is employed prevalently in a semiconductor device fabricating process. The cleaning apparatus carries workpieces, such as semiconductor wafers or glass substrate for LCDs, (hereinafter referred to as "wafers") sequentially to cleaning tanks respectively containing chemical liquids and rinsing liquids for cleaning and the like. A known cleaning apparatus shown in FIG. 13 has a cleaning tank c having an inner tank a containing a cleaning liquid L in which wafers W are immersed and an outer tank b surrounding an upper end part of the inner tank a. A circulation line f connecting cleaning liquid supply nozzles d disposed in a lower part of the inner tank a and a drain port e formed in a bottom wall of the outer tank b is provided, for example, with an air bellows circulating pump g, a damper h and a filter i. Wafers W held on a wafer boat j are immersed in the cleaning liquid L contained in the inner tank a of the cleaning apparatus, the cleaning liquid L is supplied from a cleaning liquid source into the inner tank a so that the cleaning liquid L overflows the inner tank a into the outer tank b. The cleaning liquid L overflowed into the outer tank b is filtered and circulated. The wafers W are thus cleaned.
As the cleaning liquid is used repeatedly, contaminative particles, such as particles removed from the wafers W, are accumulated in the cleaning liquid. The wafers subjected to cleaning in the cleaning liquid are contaminated, the yield of the cleaning process is reduced and the cleaning performance of the cleaning apparatus is reduced if the particle concentration of the cleaning liquid exceeds a predetermined level.
As shown in FIG. 13, a branch line k for quality testing is connected to a part of the circulation line f on the discharge side of the circulating pump g, a testing means, for example, a particle counter m, is connected to the branch line k, and the discharge side of the particle counter m is connected to the outer tank b. A portion of the cleaning liquid L contained in the inner tank a is sampled and the number of particles contained in the sample cleaning liquid is measured to monitor the number of particles contained in the predetermined quantity of cleaning liquid L. In FIG. 13, indicated at n is a shutoff valve placed in the circulation line f, at p is a drain pipe connected to a drain port q formed in the bottom wall of the inner tank a, and at r is a drain valve placed in the drain pipe q.
Since the circulating pump g is of an air bellows type, the flow rate of the sample cleaning liquid is unstable and, consequently, accurate measurement of particles cannot be achieved. Since the circulating pump g serves also as means for supplying the cleaning liquid to the particle counter m, the cleaning liquid is supplied at a flow rate exceeding the ability of the particle counter m. Furthermore, since the measurement of particles uses the sample cleaning liquid sampled at a part of the circulation line f on the discharge side of the circulating pump g, the particle counter m adds the number of particles produced by the circulating pump g to the number of particles originally contained in the cleaning liquid and, consequently, the number of particles originally contained in the cleaning liquid cannot accurately be measured.